Method for forming electrode for electrical connections to oxide super-conductor

ABSTRACT

A metal electrode formed on an oxide superconductor for electric connection to the oxide superconductor, includes a first layer of Ag in direct contact with the oxide superconductor, and a second layer formed on the first layer. The second layer is formed of noble metal excluding Ag. The metal electrode can be formed by forming a first layer of Ag to cover a whole surface of the oxide superconductor layer, and forming a second layer of noble metal excluding Ag, to cover a whole surface of the first layer, thereby to form a double metal layer, and patterning the double metal layer so as to form a metal electrode composed of the double metal layer.

This is a division of application Ser. No. 07/585,548, filed Sep. 20,1990 now U.S. Pat. No. 5,147,849.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a superconductor device using asuperconductor of compound oxide, and more specifically to an electrodefor electric connection to a compound oxide superconductor which can beeffectively implemented in a superconductor device using the compoundoxide superconductor, as well as a method for forming the sameelectrode.

2. Description of Related Art

In applications of various types of compound oxide superconductors(called simply "oxide superconductor" hereinafter), superconductorelectronic devices and superconductor wirings for electronic devices areones of fields most hopefully expected to be put into practical use.Josephson devices, SQUIDs, superconductor transistors and superconductorcircuit wirings formed of oxide superconductors have been alreadyreported.

In general, superconductor devices include superconductive conductors orwirings which allow a so-called superconducting current to flowtherethrough. However, in all of superconductor circuits and devices,the superconductive conductors or wirings have to be electricallyconnected to circuits or devices which operate under a normal conductioncondition.

For this purpose, in the above mentioned superconductor devices, anelectrical connection has been realized by using a thin metal wire suchas an Au which is called a "bonding wire". If the superconductor is ametal superconductive material, the bonding wire can be fixed andelectrically connected directly to a portion of the metalsuperconductor. However, if the superconductor is an oxidesuperconductive material, it is difficult to fix or secure the bondingwire to a portion of the oxide superconductor. In the case of the oxidesuperconductor, therefore, a metal electrode has been deposited on aportion of the superconductor by means of vacuum evaporation of noblemetal such as Au (gold), and thereafter, a bonding wire has been fixedand electrically connected to the metal electrode thus formed on theoxide superconductor. Since the noble metal typified by Au is very lowin reactivity, it will not give an adverse effect to the oxidesuperconductor. In addition, even if the noble metal typified by Au isin contact with air, it is hardly oxidized. In this point, the noblemetal typified by Au is suitable for the electrode for the oxidesuperconductor.

However, the noble metal typified by Au does not have a good adhesion orbonding property to the oxide superconductor, and therefore, a contactresistance has often become large. Therefore, the superconductor devicein which only a very small amount of electric current is flowed hasbecome unstable in operation, and cannot often exert an expectedperformance.

Furthermore, when a metal electrode is formed on a portion of an oxidesuperconductor, after a metal film is deposited on an oxidesuperconductor thin film, the metal film is patterned. It has been anordinary practice to perform the patterning by using a photolithography.

The following is one example of a "lift-off" process for forming a metalelectrode on a thin film of oxide superconductor.

First, a thin film of oxide superconductor is formed on a substrate,which has been properly selected dependently upon the kind of an oxidesuperconductor to be formed. For example, substrate is formed of MgO. Inaddition, the film of oxide superconductor is deposited by means ofsputtering, MBE (molecular beam epitaxy), CVD (chemical vapordeposition) or other suitable process.

Then, a photoresist layer is deposited on the thin film of oxidesuperconductor, and patterned so that an opening for allowing depositionof metal electrode is formed in the deposited photoresist layer. In theopening of the patterned photoresist layer, the thin film of oxidesuperconductor is exposed.

Furthermore, metal is deposited by, for example, vacuum evaporation, sothat the metal is deposited directly on the thin film of oxidesuperconductor exposed in the opening of the pattern photoresist layer.

Thereafter, the photoresist layer is removed, so that the metal layerdeposited on the photoresist layer is removed together. Thus, the metallayer remains only on a position of the thin film of oxidesuperconductor corresponding to the opening of the photoresist layer.Namely, a metal electrode having a configuration corresponding to theopening of the photoresist layer is formed on the thin film of oxidesuperconductor.

However, the above mentioned conventional metal electrode forming methodis disadvantageous in that, since the photoresist layer is depositeddirectly on a surface of the thin film of oxide superconductor, aninterfacial reaction occurs, and therefore, the characteristics of theoxide superconductor is deteriorated. In addition, in the process of thephotolithography, since the oxide superconductor is in contact with analkaline developing liquid and a cleaning water, the characteristics ofthe oxide superconductor is further deteriorated.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a metalelectrode for electric connection to an oxide superconductor, which hasovercome the above mentioned defect of the conventional one and whichhas a good adhesion or bonding property to the oxide superconductor anda sufficiently low contact resistance.

Another object of the present invention is to provide a method forforming a metal electrode for electric connection to an oxidesuperconductor, without deteriorating the characteristics of the oxidesuperconductor.

The above and other objects of the present invention are achieved inaccordance with the present invention by a metal electrode formed on anoxide superconductor for electric connection to the oxidesuperconductor, the metal electrode including a first layer of Ag(silver) in direct contact with the oxide superconductor, and a secondlayer formed on the first layer, the second layer being formed of noblemetal excluding Ag.

According to another aspect of the present invention, there is provideda method for forming a metal electrode on an oxide superconductor layerfor electric connection to the oxide superconductor layer, comprisingthe steps of forming a first layer of Ag to cover a whole surface of theoxide superconductor layer, and forming a second layer of noble metalexcluding Ag, to cover a whole surface of the first layer, thereby toform a double metal layer, and patterning the double metal layer so asto form a metal electrode composed of the double metal layer.

As seen from the above, the metal electrode in accordance with thepresent invention for electric connection to the oxide superconductor ismainly characterized in that the metal electrode is composed of twolayers, namely, a first layer of Ag in direct contact with the oxidesuperconductor, and a second layer formed on the first layer and formedof noble metal excluding Ag. Since Ag is remarkably low in reactivity tooxide superconductors. Ag will never give an adverse influence to theoxide superconductor. In addition, Ag has a low contact resistance withoxide superconductors and an excellent adhesion or bonding property tooxide superconductor. This is a characteristics peculiar or inherent toAg. The electrode in accordance with the present invention utilizes thischaracteristics of Ag.

On the other hand, Ag is easily oxidized in air. In this aspect, Ag isnot preferable as an electrode material. However, noble metal such asAu, excluding Ag, is hardly oxidized in air. Therefore, in this aspect,the noble metal excluding Ag is preferable as an electrode material.However, the noble metal excluding Ag is poor in bonding property tooxide superconductor, so that a substantial contact resistance oftenoccurs.

Thus, in order to realize a metal electrode which has a good bondingproperty to oxide superconductor without adversely affecting the oxidesuperconductor, and which is never easily oxidized in air, the metalelectrode in accordance with the present invention is composed of adouble metal layer having such a construction that a portion in directcontact with an oxide superconductor is formed of an Ag layer and aportion in contact with air is formed of a layer of noble metalexcluding Ag, for example, Au or Pt (platinum).

Preferably, a thickness of the Ag layer and a thickness of the noblemetal layer formed on the Ag layer are in a range of 0.01 μm to 1 μm andin a range of 0.05 μm to 1 μm, respectively. If the thickness of the Aglayer is less than 0.05 μm, the Ag layer has no effect of protecting theoxide superconductor. On the other hand, even if the thickness of the Aglayer is greater than 1 μm, the effect of protecting the oxidesuperconductor is not increased, and rather, a long time becomesrequired for removal of unnecessary portion of the metal layer after aphotolithography process. Similarly, if the thickness of the noble metallayer is less than 0.05 μm, the noble metal layer has no effect ofprotecting the Ag layer, and is not sufficient to allow the electrode tofunction. On the other hand, even if the thickness of the noble metallayer is greater than 1 μm, the effect of protecting the Ag layer is notincreased, and rather, a long time becomes required for removal ofunnecessary portion of the metal layer after a photolithography process.Therefore, the above mentioned ranges of thickness are preferred.

The method in accordance with the present invention is characterized bycovering a whole surface of an oxide superconductor layer with a doublemetal layer composed of a Ag sub-layer and another sub-layer of noblemetal excluding Ag, and then, patterning the double metal layer into aform of an electrode. In this method in accordance with the presentinvention, since the oxide superconductor layer is in direct contactwith neither photoresist nor developing liquid, the characteristics ofthe oxide superconductor will never be deteriorated by the photoresistor the developing liquid.

The metal electrode in accordance with the present invention can befabricated by using a deposition process which has been used forfabrication of conventional electrodes. In this connection, it ispreferred that after formation of the double metal layer, the metalelectrode in accordance with the present invention is heated so as toimprove an adhesion or bonding property between the Ag layer and theoxide superconductor layer. Preferably, the heat treatment is performedin a range of 300° C.to 580° C. If the heating temperature is less than300° C., the heating treatment is not so effective in improving thebonding property. On the other hand, if the heating temperature isgreater than 580° C., a reaction layer is formed, so that thecharacteristics of the oxide superconductor is deteriorated. It is moreeffective if the heating is performed in atmosphere of oxygen. In thiscase, the heating processing is preferably performed after completion offormation of the second layer of noble metal excluding Ag, in order toprotect the Ag layer from oxidation.

Furthermore, in the method of the present invention, the etching afterphotolithography is preferably performed by a dry etching process, forexample, an ion beam etching using inert gas such as Ar, an ECR(electron cycroton resonance) etching, an RF (radio frequency) plasmaetching, etc. These etching processes are very preferable, since aphysical etching is realized by charged particles without chemicalreaction, and therefore with less influence to the oxide superconductor.

The above and other objects, features and advantages of the presentinvention will be apparent from the following description of preferredembodiments of the invention with reference to the accompanyingdrawings. However, the examples explained hereinafter are only forillustration of the present invention, and therefore, it should beunderstood that the present invention is in no way limited to thefollowing examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic perspective view illustrating a structure of ametal electrode in accordance with the present invention for an electricconnection to an oxide superconductor;

FIGS. 2A and 2B illustrate a method of measuring a contact resistance inthe metal electrode for the oxide superconductor; and

FIGS. 3A, 3B, 3C, 3D, 3E and 3F illustrate one embodiment of the processin accordance with the present invention for fabricating the metalelectrode in accordance with the present invention for an electricconnection to an oxide superconductor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A metal electrode in accordance with the present invention having aconfiguration as shown in FIG. 1 and a conventional metal electrode wereformed on various types of oxide superconductor layers, and comparisonwas performed about characteristics of the electrodes. As shown in FIG.1, on a thin film 1 of an oxide superconductor formed on an insulativesubstrate 2, there is formed a metal electrode 3 constituted of an lowermetal sub-layer 5 of Ag and an upper metal sub-layer 6 of Au.

In examples explained hereinafter, a contact resistance in a metalelectrode for electric connection to an oxide superconductor wasmeasured by using a so-called "three-terminal method" and a so-called"four-terminal method" in combination.

First, as illustrated in FIG. 2A, conventional electric contacts C₁ andC₂ and an electric contact or electrode C₃ in accordance with thepresent invention are formed on the oxide superconductor thin film 1. Anelectric current I is flowed between the contacts C₁ and C₃ and ismeasured by an ammeter 7. On the other hand, a voltmeter 8 is connectedbetween the contacts C₂ and C₃ so that an electric voltage V between thecontacts C₂ and C₃ is measured by the voltmeter 8. Here, it is assumedthat contact resistances at the contacts C₁, C₂ and C₃ are r₁, r₂ andr₃, respectively, and an equivalent resistance in a portion of thesuperconductor thin film 1 between the contacts C₂ and C₃ is R. It isalso assumed that an internal impedance of the voltmeter 8 is infinite.

Under the above mentioned arrangement, the voltmeter 8 measures avoltage drop V occurring when the current I flows in series through theequivalent resistance R of the superconductor thin film 1 and thecontact resistance r₃. Therefore,

    r.sub.3 =(V/I)-R                                           (1)

Furthermore, as illustrated in FIG. 2B, a conventional electric contactC₄ is formed on the superconductor thin film 1. An electric current Iais flowed between the contacts C₁ and C₄ and is measured by the ammeter7. On the other hand, an electric voltage Va between the contacts C₂ andC₃ is measured by the voltmeter 8. Here, it is assumed that a contactresistance at the contact C₄ is r₄.

In this case, the voltmeter 8 measures a voltage drop Va occurring whenthe current Ia flows in series through only the equivalent resistance Rof the superconductor thin film 1. Therefore,

    Va=Ia.R                                                    (2)

Accordingly, the following equation can be derived from the aboveequations (1) and (2).

    r.sub.3 =(V/I)-(Va/Ia)                                     (3)

Thus, the contact resistance r₃ in the electric contact or electrode C₃to the oxide superconductor in accordance with the present invention canbe measured by a sequential measurement of the "three-terminal method"and the "four-terminal method", without being influenced by values ofr₁, r₂ and r₄, and R.

EXAMPLE 1

A metal electrode was formed on an oxide superconductor thin film inaccordance with the present invention. A process for formation of themetal electrode will be explained with reference to FIGS. 3A to 3F.

As shown in FIG. 3A, an oxide superconductor thin film 1 of Y₁ Ba₂ Cu₃O_(x) (6<x≦7) having a thickness of 0.5 μm was formed on amonocrystalline substrate 2 of MgO (100) by sputtering. The oxidesuperconductor thin film 1 of Y₁ Ba₂ Cu₃ O_(x) thus formed had acritical temperature Tc of 90K.

As shown in FIG. 3B, an Ag layer 5 having a thickness of 0.15 μm wasdeposited on a whole surface of the oxide superconductor thin film 1 ofY₁ Ba₂ Cu₃ O_(x) by means of a vacuum evaporation process. In addition,an Au layer 6 having a thickness of 0.10 μm was also deposited on awhole surface of the Ag layer 5 by means of a vacuum evaporationprocess, as shown in FIG. 3C.

The condition for the above vacuum evaporations was as follows:

    ______________________________________                                        Heating of the substrate                                                                           No heating                                               Degree of vacuum     1 to 3 × 10.sup.-6 torr                            Deposition rate      2 to 3 Å/second                                      ______________________________________                                    

After formation of the Au layer 6, the substrate was heated at atemperature of 300°C. for 10 minutes in an atmospheric pressure ovensupplied with a flow of oxygen.

Thereafter, as shown in FIG. 3D, a photoresist layer 4 was formed on theAu layer 6. Then, as shown in FIG. 3E, the photoresist layer 4 waspatterned to form a photoresist pattern 30 at a position on which ametal electrode is to be formed.

An exposed portion of the double metal layer was etched by means of anAr ion beam etching process using a Kaufman type ion gun. The etchingwas terminated when the Ag layer of the exposed double metal layer wascompletely removed. A remaining resist layer was removed in an ashingprocess by using O₂ plasma. Thus, a metal electrode 3 constituted of theAg layer 5 and the Au layer 6 was formed as shown in FIG. 3F.

In addition, for comparison, a metal electrode consisting of only asingle Au layer having a thickness of 0.25 μm and having the sameconfiguration as that of the metal electrode 3 was formed, in accordancewith the conventional method explained hereinbefore, on an oxidesuperconductor thin film of Y₁ Ba₂ Cu₃ O_(x) having the samecharacteristics.

The oxide superconductor thin film of Y₁ Ba₂ Cu₃ O_(x), on a surface ofwhich the metal electrode was formed in accordance with the presentinvention, had the critical temperature Tc of 90K without change evenafter formation of the metal electrode. In the oxide superconductor thinfilm of Y₁ Ba₂ Cu₃ O_(x) formed with the Au electrode in accordance withthe conventional process, the critical temperature Tc after formation ofthe metal electrode dropped from 90 K. to 80 K.

In addition, a contact resistance between the respective electrodes andthe oxide superconductor was measured at a temperature of 77.3 K. Thecontact resistance of the electrode in accordance with the presentinvention was 5.6×10⁻⁸ Ωcm². On the other hand, the contact resistanceof the Au single layer electrode in accordance with the prior art was6.4×10⁻⁵ Ωcm².

Furthermore, the Au electrode formed in accordance with the prior artwas poor in the bonding property between the electrode and the oxidesuperconductor thin film, and easily peeled off. However, none of thedefects was found in the Ag/Au electrode formed in accordance with thepresent invention.

EXAMPLE 2

In a process similar to that of the Example 1, an Au/Ag electrode wasformed on an oxide superconductor thin film of Bi₂ Sr₂ Ca₂ Cu₃ O_(y)(7≦y≦10) having a thickness of 0.5 μm. Then, a critical temperature Tcof the oxide superconductor thin film of Bi₂ Sr₂ Ca₂ Cu₃ O_(y) wasmeasured before and after formation of the electrode. In addition, an Auelectrode was formed in accordance with the conventional method on anoxide superconductor thin film of Bi₂ Sr₂ Ca₂ Cu₃ O_(y) having the samecharacteristics, and similarly, a critical temperature Tc of the oxidesuperconductor thin film was measured before and after formation of theelectrode. The result is shown in the following table.

    ______________________________________                                                    Before      After                                                             formation of                                                                              formation of                                                      electrode   electrode                                             ______________________________________                                        Invention     105 K         105 K                                             Comparative   105 K          85 K                                             ______________________________________                                    

In the oxide superconductor thin film having the Au electrode formed inaccordance with the conventional process, not only the criticaltemperature Tc of the oxide superconductor thin film dropped afterformation of the metal electrode, but also the electrode was poor in thebonding property and easily peeled off.

The following is the contact resistance between the respectiveelectrodes and the oxide superconductor which was measured at atemperature of 77.3 K. in the same manner as that in the Example 1.

    ______________________________________                                                      Invention Comparative                                           ______________________________________                                        Contact resistance                                                                            6.3 × 10.sup.-8                                                                     7.2 × 10.sup.-5                             (Ω cm.sup.2)                                                            ______________________________________                                    

EXAMPLE 3

In a process similar to that of the Example 1, an Au/Ag electrode wasformed on an oxide superconductor thin film of Tl₂ Ba₂ Ca₂ Cu₃ O_(z)(7≦z≦10) having a thickness of 0.5 μm. Then, a critical temperature Tcof the oxide superconductor thin film of Tl₂ Ba₂ Ca₂ Cu₃ O_(z) wasmeasured before and after formation of the electrode. In addition, an Auelectrode was formed in accordance with the conventional method on anoxide superconductor thin film of Tl₂ Ba₂ Ca₂ Cu₃ O_(z) having the samecharacteristics, and similarly, a critical temperature Tc of the oxidesuperconductor thin film was measured before and after formation of theelectrode. The result is shown in the following table.

    ______________________________________                                                    Before      After                                                             formation of                                                                              formation of                                                      electrode   electrode                                             ______________________________________                                        Invention     114 K         114 K                                             Comparative   114 K          98 K                                             ______________________________________                                    

In the oxide superconductor thin film having the Au electrode formed inaccordance with the conventional process, not only the criticaltemperature Tc of the oxide superconductor thin film dropped afterformation of the metal electrode, but also the electrode was poor in thebonding property and easily peeled off.

The following is the contact resistance between the respectiveelectrodes and the oxide superconductor which was measured at atemperature of 77.3 K. in the same manner as that in the Example 1.

    ______________________________________                                                      Invention Comparative                                           ______________________________________                                        Contact resistance                                                                            6.8 × 10.sup.-8                                                                     7.4 × 10.sup.-5                             (Ω cm.sup.2)                                                            ______________________________________                                    

As seen from the above, the electrode composed of a normal conductor inaccordance with the present invention for electrical connection to anoxide superconductor thin film is excellent in the bonding property tothe oxide superconductor as compared with the conventional ones, and issmaller in contact resistance than the conventional ones. Therefore, ifthe electrode in accordance with the present invention is used in asuperconductor device, it is expected that noise is decreased andperformance is improved.

In addition, the method of the present invention makes it possible toform a metal electrode on an oxide superconductor layer withoutdeteriorating the characteristics of the oxide superconductor layer.Therefore, the method of the present invention can be expected tofacilitate application of oxide superconductors to superconductordevices including Josephson devices and superconductor transistors.

The invention has thus been shown and described with reference to thespecific embodiments. However, it should be noted that the presentinvention is in no way limited to the details of the illustratedstructures but changes and modifications may be made within the scope ofthe appended claims.

We claim:
 1. A method for forming a metal electrode on an oxidesuperconductor layer for electric connection to the oxide superconductorlayer, comprising the steps of forming a first layer of Ag to cover awhole surface of the oxide superconductor layer, and forming a secondlayer of noble metal excluding Ag, to cover a whole surface of saidfirst layer, thereby to form a double metal layer, and patterning saiddouble metal layer so as to form on the oxide superconductor layer ametal electrode composed of said double metal layer.
 2. A method claimedin claim 1 wherein said second layer is made of Au.
 3. A method claimedin claim 2 wherein a thickness of said first layer of Ag is in a rangeof 0.01 μm to 1 μm and a thickness of said second layer of Au is in arange of 0.05 μm to 1 μm.
 4. A method claimed in claim 3 wherein afterformation of said double metal layer, a heating treatment is performedin order to improve a bonding property between said double metal layerand said oxide superconductor layer.
 5. A method claimed in claim 4wherein said heating treatment is performed under an atmosphere ofoxygen in a temperature range of 300° C. to 580° C.
 6. A method claimedin claims 4 wherein after said heating treatment, a resist pattern isdeposited on said double metal layer, and then, said double metal layeris selectively removed by a physical dry etching process using saidresist pattern as a mask, so that said metal electrode is formed.
 7. Amethod claimed in claim 6 wherein said heating treatment is performedunder an atmosphere of oxygen in a temperature range of 300° C. to 580°C.
 8. A method claimed in claim 1 wherein a thickness of said firstlayer of Ag is in a range of 0.01 μm to 1 μm and a thickness of thesecond layer of noble metal excluding Ag is in a range of 0.05 μm to 1μm.
 9. A method claimed in claim 1 wherein after formation of saiddouble metal layer, a heating treatment is performed in order to improvea bonding property between said double metal layer and said oxidesuperconductor layer.
 10. A method claimed in claims 1 wherein afterformation of said double metal layer, a resist pattern is deposited onsaid double metal layer, and then, said double metal layer isselectively removed by a physical dry etching process using said resistpattern as a mask, so that said metal electrode is formed.